1. Field of the Invention
The present invention relates to a method for calibrating by calculation appliances for the measurement of force or moment which comprise a fixed base, a load platform on which is applied the force or the moment to be measured, and a measuring element inserted at one end in the fixed base and at the other end in the load platform and instrumented as a sensor.
The invention is also directed to appliances which are calibrated in this manner.
Appliances of this type are employed in particular for weighing (scales for weighing persons, scales for weighing food products, scales of other types) or for the measurement of torque.
2. Description of the Prior Art
In these appliances, the measuring element which usually consists of a bar adapted to carry strain gages is subjected in practice not only to the load (force or moment) to be measured but also to parasitic moments which may result either from imperfections of the equipment (related, for example, to the bar, to the insertion of this latter, to the strain gages and to the arrangement of these latter on the bar) or from an imperfection in the mode of application of the load to be measured (for example a load placed on the platform of a weighing device produces parasitic twisting or bending moments according to its position on the platform).
In these devices, the gages carried by the bar are connected electrically in a measuring bridge for delivering a signal which is sensitive to these parasitic moments. Said signal therefore has parasitic components which impair the accuracy of measurement of the force or moment to be measured.
In order to overcome this drawback, a number of technical solutions are already known and applied.
In a first known solution, the measuring element is adjusted by removal of material at a judiciously chosen point of the strain gages in order to modify the angle formed between the electrical direction of the gages and the longitudinal axis of the measurement body or in other words of the bar.
This solution has a disadvantage in that it cannot readily be expressed in an automatic application algorithm, calls for empirical interpretation of faults, highly qualified personnel and very costly means.
In accordance with another method, there are added to the strain-gage bridge for measurement of the load (force or moment) to be measured additional or adjusting gages for correcting the effect of parasitic loads.
In accordance with French Patent No. 2,554,229, provision is made for an arrangement of strain gages on the bar of a weighing device which makes it possible to read on the one hand a signal which is mainly proportional to the weight applied on the bar but is affected by parasitic components generated by the moments along the axis of the bar and along an axis perpendicular to this latter and on the other hand two signals which are mainly proportional to each of these two moments. It is then possible to reinject in the signal which is mainly proportional to the weight part of the signals which are mainly proportional to the two moments aforesaid, namely that part which is necessary for compensating and cancelling the parasitic components of the main signal.
The solutions described in the foregoing are based on the same approach which consists in materially modifying the measuring element, namely the bar with its strain gages, so as to make said element sensitive solely to the load (force or moment) to be measured and insensitive to parasitic loads. In consequence, these solutions are all subject to the same drawback since it is necessary after manufacture to take each weighing apparatus in hand again for a modification which is specific to that apparatus (such as, for example, welding of additional components, cutting of tracks of certain gages, and so on). These operations are both time-consuming and costly.
The aim of the present invention is to circumvent all these disadvantages by means of a method of calibration which no longer calls for any material intervention of the measuring element after manufacture of this latter and which can be fully automated.